1. Field of the Invention
The present invention relates generally to a combined polarizer and diffraction grating to polarize and further control light, such as by reducing zero order back reflection.
2. Related Art
Diffraction gratings are a periodic structure of dielectric material with a period (p) greater than about half the wavelength (λ) of incident light, or p≧˜λ/2. The diffraction grating scatters the normally incident light at discrete angles or directions in accordance with mλ=p sin θ, where m is the order and θ is the angle with respect to normal from the diffraction grating. Thus, different wavelengths are reflected or scattered at different angles.
Wire grid polarizers are a periodic structure of conductive elements with a length greater than the wavelength and a period less than about half the wavelength of the incident light, or p≦˜λ/2. Examples of wire grid polarizers are shown in U.S. Pat. Nos. 6,288,840; 6,243,199 and 6,122,103. Wire grid polarizers have been proven to be effective for visible light (˜400-700 nm, or ˜0.4-0.7 microns or μm) and their use demonstrated as polarizers and beam splitters in optical imaging systems. For example, see U.S. Pat. Nos. 6,234,634 and 6,447,120. Composite wire-grid polarizers have been proposed in which the wires include alternating layers of dielectric and conductive layers. For example, see U.S. Pat. Nos. 6,532,111; 6,665,119; 6,788,461 and 7,113,335. Such polarizers, however, can also back reflect a portion of the incident light resulting in a ghost image, indicated at 500 in FIG. 13. One solution has been to tilt or angle the polarizer to direct the back reflection out of or away from the optical path, as shown in FIG. 14. Tilting the polarizer, however, can take-up valuable space in a compact design, and can cause unwanted astigmatism.